1. Field of the Invention
The present invention relates to an intermediate-frequency circuit of a television tuner, and more specifically relates to an intermediate-frequency circuit formed of a integrated circuit of a television tuner.
2. Description of the Related Art
Referring to FIG. 6, an intermediate-frequency circuit of a conventional type television tuner will be described below. An integrated circuit 21 includes a mixer 22, an intermediate-frequency amplifier 23 and in addition, a local oscillator (not shown). The mixer 22 is formed as a balanced type mixer provided with balanced output terminals, a television signal RF of a receive channel selected by an external double-tuned circuit is input to the mixer 22 and a signal oscillated from the local oscillator is input to the mixer 22. An intermediate-frequency signal after frequency conversion is output from the mixer 22.
The intermediate-frequency amplifier 23 is formed as a balanced type amplifier provided with balanced input terminals. The balanced output terminal of the mixer 22 and the balanced input terminal of the intermediate-frequency amplifier 23 are connected via each connecting capacitative element 24 in the integrated circuit 21 and an intermediate-frequency signal is input to the intermediate-frequency amplifier 23. In the integrated circuit 21, two external terminals 21a and 21b respectively connected to the balanced output terminals of the mixer 22 are also provided and an intermediate frequency tuning circuit 25 is connected between the external terminals 21a and 21b. The intermediate frequency tuning circuit 25 is provided with a tuning characteristic that the intermediate frequency turning circuit tunes to a substantially intermediate frequency between a video intermediate carrier Fp and a chrominance subcarrier Fc as shown in FIG. 7.
As described above, a television signal of an adjacent channel is removed by providing the intermediate frequency tuning circuit 25 connected between the external terminals 21a and 21b between the mixer 22 and the intermediate-frequency amplifier 23, only a television signal of a receive channel is input to the intermediate-frequency amplifier 23 and an intermediate-frequency signal IF output from the intermediate-frequency amplifier 23 is input to an image detector (not shown) via a SAW filter (not shown) provided outside the integrated circuit 21 (as well as other components).
However, in this configuration, a television signal of an adjacent channel cannot be sufficiently damped and a television signal of a receive channel is disturbed by a video intermediate-frequency signal and a voice intermediate-frequency signal respectively included in the adjacent channel. For a measure against this, a trap circuit for damping a video intermediate-frequency signal and a voice intermediate-frequency signal respectively included in an adjacent channel has only to be provided in the tuning circuit 25, however, then, there occurs a problem that the intermediate frequency tuning circuit 25 and the trap circuit have an effect upon each other and it becomes difficult to set a tuning frequency and a trapping frequency.